I have a T206H with a Lycoming TSIO-540-AJ1A with 550 hours. I just had it in annual in Oct. and all the compressions were 70+. In the last +/- 15 hours I have noticed oil consumption increased to about 1 qt. every 4 hours. Prior it was using maybe 1 every 15ish. I naturally thought I had an oil leak. So off to the shop it went. They are telling me that the compression is all still good, but I have a cylinder pressurizing the crankcase and blowing the oil out of the breather. They said that they apply 80 PSI to each cylinder and on number 5 they could hear it blowing down into the crankcase. The plugs on all cylinders including 5 look perfect. Temps and other engine indications, perfect. They say I have a stuck ring. Options are to soak it in Marvel Mystery Oil OR rebuild the cylinder. So how does this work? Can you have good compression and excessive blowby? Seems counter-intuitive. Isn't pressure, pressure? Why would a ring just get stuck? Does Marvel work, or am I wasting labor dollars? They are quoting approx. 10 hours labor and about $1400 for the shop to rework the cylinder, rings, valves, etc. Has anyone seen this before? Thoughts?